@UNIX4ALL: are you using FreeBSD's AGPGART driver or the builtin NvAGP driver? In the former case, does the SMP problem reproduce with NvAGP? Also, does the behavior change if you build the kernel without optimizations (i.e. with the default flags)?
I use booth, and hang with NvAGP too, moving away make.conf (for defaults optimizations) make the same situation
Post #39 have information about my hardware and software versions.